Method for Lubricating Milling Material

ABSTRACT

Disclosed is a method for rolling milling material, especially for hot wide-strip rolling in a finishing train or a continuous casting and rolling plant, in which a lubricant is applied directly to the surface of the working rollers or indirectly to the surface of the support rollers, the lubricant then being transferred to the surface of the working rollers, before the milling material is fed into the roller gap of a roll stand. A highly adhesive lubricant film forms on the surface of the working roller, said lubricant film resulting in a reduction of friction in the roller gap as an intermediate layer between the roller and the milling material. According to the inventive method, the lubricant is applied along the entire length of the milling material such that the lubricating effect comes into play along the entire length of the milling material.

The invention concerns a method for the rolling of rolling stock,especially for the rolling of hot-rolled wide strip in a finishing trainor a continuous casting installation, in which a lubricant is applieddirectly to the surface of the work rolls or indirectly to the surfaceof the backup rolls and is then transferred to the surface of the workrolls, before the rolling stock enters the roll gap of a rolling stand,wherein a strongly adherent lubricant film forms on the surface of thework rolls and leads to reduction of friction in the roll gap as anintermediate layer between the roll and the rolling stock.

The use of roll gap lubrication in heavily loaded hot-rolling finishingtrains is well known. However, the lubrication is turned on with somedelay at the leading end of the strip after the pass has started in thegiven stand and is turned off a certain amount of time before thetrailing end of the strip exits the stand. Depending on the lubricatingaction, a procedure of this type has an unfavorable effect on therolling process.

DE 21 05 975 discloses a device for automatically feeding oil to ahot-rolling mill stand through which rolling stock is passing. Thedevice comprises an oil spraying device for the rolling stand, means forfeeding oil to the spraying device, means for initiating operation ofthe oil feed device shortly after the leading end of the rolling stockpasses through the rolling stand, and means for terminating operation ofthe oil feed device shortly before the trailing end of the rolling stockreaches the rolling stand.

The document U.S. Pat. No. 6,266,985 B1 describes a method in which alubricant is applied to the corners and ends of the rolling stock thatis to be hot-rolled.

DE 20 22 923 discloses a method and a device for applying a lubricant torolling stock in a hot-rolling strip mill, wherein lubricant andatomizing air are delivered to several spray nozzles in a four-highrolling stand equipped with two work rolls and two backup rolls. Thespray nozzles are arranged across the width of at least one of thebackup rolls. As soon as the rolling stock enters the work rolls, thelubricant is applied to the backup rolls, from which it is transferredto the work rolls. The flow of lubricant and atomizing air to thenozzles is cut off as soon as the rolling stock leaves the work rolls.

The starting time for the lubricant feed is the start of the pass in thegiven stand. A disadvantage of this method and this device is that aclosed lubricant film must first form on the surface of the backup rolland then be transferred to the surface of the work roll in the gapbetween the backup roll and work roll. From the work roll, the lubricantenters the roll gap. It takes too much time for the lubricant to reachthe rolling stock (several seconds), depending on the peripheral speedand diameter of the roll. In a method of this type and a device of thistype, the leading end of the strip is not supplied with lubricant as itenters the roll gap, which temporarily results in higher rolling forces.

Fundamentally, rolling is not possible without friction in the roll gap,since sufficient friction is required to bite the rolling stock and pullit through the roll gap. It is always necessary to ensure that theso-called bite and pass conditions are met. With respect to the minimumrequired friction, the bite condition, which describes the conditions atthe leading end of the rolling stock, is much more critical than thepass condition. In the pass condition, the roll gap is completely filledwith rolling stock.

If one of the two conditions or both conditions are not satisfied,slipping occurs, i.e., slip between the roll and the rolling stock.

In addition to the coefficient of friction, which describes the frictionitself, the bite angle and the rolling speed play a role. The bitecapacity decreases with increasing rolling speed. With respect to thebite angle, which depends on the thickness of the entering rollingstock, the relative thickness reduction during rolling, and the diameterof the rolls, it can be said that it must always be smaller than thefriction angle to allow slip-free rolling. With the usual draftdistribution, the bite angle in finishing trains decreases from thefirst to the last stand. Accordingly, the greatest bite problems, mainlyslipping at the leading end of the rolling stock, occur in the firstfinishing stand.

Since the bite condition at the leading end of the rolling stock is moredifficult to meet than the pass conditions in the rolling stock, theroll gap lubrication is adjusted in such a way that the lubrication isnot turned on until after the initial pass has started in the givenstand and is turned off a certain amount of time before the rollingstock exits the stand. This allows the work rolls to continue rollingsufficiently long for the lubricant present on the surface of the rollsto burn off completely. In this way, the bite at the leading end of thefollowing piece of rolling stock is not jeopardized.

As a result of the lubrication strategy described above, two differentload levels develop within the rolling stock, provided that the prestripor the thin slab enters the finishing train with homogeneous propertiesover the length and without speed-up: a high level at the leading endand the trailing end of the rolling stock (without lubricating action)and a low rolling force level in the remainder of the rolling stock(active lubrication).

As a result of the reduction of the rolling force during activelubrication, which can amount to 50% or more, there is a change in thedeflection of the work rolls. This simultaneously results in a change inthe state of flatness of the rolling stock (prominent in the rearstands) and in the crown of the rolling stock (prominent in the frontstands).

However, if work roll bending is available as an adjusting mechanism inthe given stand, then in the case of large rolling force reductions, theadjustment limits are more frequently reached. The result is unstabletravel of the rolling stock, especially in the rolling of thin slabs.

It is customary to limit the rolling force reduction to a value of about20%.

Therefore, the objective of the invention is to specify a method forapplying a lubricant to the surface of a work roll or backup roll, whichimproves the rolling process, leads to a reduction of roll wear, andlowers the power consumption of the finishing train.

In accordance with the invention, this objective is achieved by thecharacterizing features of claim 1. Advantageous refinements of theinvention are specified in the dependent claims.

If a lubrication system that feeds lubricant from a reservoir to thework rolls or backup rolls through lines and nozzles is turned on 5-15seconds before the rolling stock enters the rolling stand, all idletimes in the lubrication system are compensated, so that a closedlubricating film always forms on the work roll even before the start ofthe pass.

The decisive advantage of the method of the invention is that a constantlevel of rolling force for the given stand is obtained over the entirelength of the rolling stock, because the lubrication is maintained overthe entire length of the rolling stock.

Due to the constancy of the rolling force, greater total rolling forcereductions can be achieved, for example, 40-50%, so that the loading ofthe rolling train is greatly reduced with respect to wear and powerconsumption. In addition, the greater rolling force reduction reduceswork roll wear and thus increases the useful life of the rolls (servicelife).

The advantages of the method of the invention are thus:

-   -   constant rolling force level;    -   no change in the state of flatness related to the rolling force;    -   no change in strip profile related to the rolling force;    -   since the forces at the leading end and the trailing end of the        rolling stock are also significantly reduced, stand vibrations        and roll damage can be even more effectively avoided;    -   this results in greater latitude for draft distributions and        thus optimized pass programs for rolling critical products;    -   since the state of flatness at the trailing end of the rolling        stock does not change due to the rolling force, incorrect        rolling of the leading end of the rolling stock can be avoided.

A prerequisite for the use of the lubrication of the invention is thatthe bite conditions at the leading end of the rolling stock are ensured.

The application of the lubricant in accordance with the invention in afinishing train or a continuous casting installation is advantageouslypossible without restriction starting with the second stand and for allsubsequent stands.

Lubricant is generally not applied in the first finishing stand due tothe great thickness of the rolling stock (large bite angle). Inaccordance with the invention, in order already to apply the lubricantin the second finishing stand, the conditions existing in the giveninstallation are taken into consideration. These include, for example

-   -   the condition of the surface of the roll after grinding,    -   the work roll diameter,    -   the work roll material,    -   the entering strip thickness,    -   the relative reduction that is adjusted,    -   the surface condition of the entering rolling stock (scale,        temperature, roughness, material, etc.),    -   the lubricant used,    -   the amount of lubricant, and    -   the rolling speed.

If the bite angle is in the critical range, the lubricating action atthe leading end of the rolling stock is upwardly limited by a smalleramount of lubricant or by a modified lubricant, i.e., a lubricant withdifferent properties.

When the method of the invention for applying lubricant to a rollsurface is used, it is advisable to use the pass program model of thefinishing train for this operation. In this regard, at least a doublingof the adaptation matrix should be provided so as not to adverselyaffect the adapted operation without roll gap lubrication. In physicalmodels, the selection of different sets of coefficients of friction—forthe lubricated and unlubricated state—is highly advantageous for thestartup, when new products are introduced, and for avoiding highadaptation values.

Every change in friction is associated with a change in the forwardslip, i.e., with a change in mass flow or a disturbance of mass flow.This type of disturbance must be stabilized by the automatic controlsystems, which can lead to considerable difficulties in critical thinstrip rolling.

Another advantage of the method of the invention is that, when constantlubrication is provided, constant forward slip is achieved, which inturn results in constant mass flow.

In ferritic rolling, the roll gap lubrication is used to have afavorable effect on the shear texture of the surface. The application ofthe lubricant in accordance with the invention produces a homogeneoussurface texture over the whole length of the rolled product.

To avoid time lags when the lubrication is turned on, the lubricationsystem is provided with a closed-circulation line with return to thelubricant tank and a switchable 3/2-way valve before the lubricant mixer(for example, a static-tube mixer).

1. Method for the rolling of rolling stock, especially for the rolling of hot-rolled wide strip in a finishing train or a continuous casting installation, in which a lubricant is applied directly to the surface of the work rolls or indirectly to the surface of the backup rolls and is then transferred to the surface of the work rolls, before the rolling stock enters the roll gap of a rolling stand, wherein a strongly adherent lubricant film forms on the surface of the work rolls and leads to reduction of friction in the roll gap as an intermediate layer between the roll and the rolling stock, wherein the lubricant is applied over the entire length of the rolling stock, and therefore the lubricating effect acts over the entire length of the rolling stock.
 2. Method in accordance with claim 1, wherein a lubrication system that feeds lubricant from a reservoir to the work rolls or backup rolls through lines and nozzles is turned on 5-15 seconds before the rolling stock enters the rolling stand.
 3. Method in accordance with claim 1, wherein the lubricant is applied in the second stand and in each subsequent stand.
 4. Method in accordance with claim 1, wherein the lubricant is applied in the third stand and in each subsequent stand.
 5. Method in accordance with claim 1, wherein the lubricant is applied in the second stand and in each desired subsequent stand.
 6. Method in accordance with claim 1, wherein the lubricant is applied in the third stand and in each desired subsequent stand. 